Lesson 08 · 60 minutes
When in Doubt, Stop
Designing a robot that fails safely
Today's goal
List every way our robot stops itself, test each one, and invent one more safety rule.
Ideas in this lesson
- A robot must assume something will go wrong
- Dead-man controls: let go and it stops
- Sensors can be a safety net (the wall stop)
- Stopping is the default answer when the robot is not sure
Sense → Think → Do
Safety is a SENSE / THINK / DO loop of its own: sense danger (a wall, a lost signal, a lost line), decide it is not sure, do the safest thing, which is usually stop.
How the session runs
- Hold the up arrow on the IR remote, then let go. The robot stops about a quarter second after the last signal. Why not keep going until told to stop?
- On the two-stick controller page, lift your thumb: that stick springs back to the middle and its part of the motion stops.
- Drive forward toward a wall (w, or the up arrow). The laser stops it about 120 mm away and the console says 'tof: wall at N mm'.
- Try driving forward again while close: it refuses. Drive backward or sideways: allowed. Why only forward?
- Start line following and lift the robot off the line. It stops after the lost-stop time.
- Start a demo, then press any remote key. Every key aborts a demo.
- In the app, start a demo: the red STOP bar sits at the bottom of every tab, and it keeps sending stop until the robot confirms.
- Group task: invent one more safety rule. What should the robot do if its battery is low? If someone picks it up?
Try this
- What is the slowest you would let a robot near a little kid? Why?
- The wall stop only looks forward. Where else could the robot crash?
- Is 'stop' always the safest thing? Think of a self-driving car on a motorway.
When it goes wrong
| What you see | What it means |
|---|---|
| Wall stop never triggers | No laser fitted, or it was not found at start-up (Sensors tab says 'not found'). |
| Robot keeps going after letting go of the remote | The receiver still sees repeats (another remote nearby?), or it was a key that runs a demo: press any key to abort. |
| STOP bar does not go away | It waits for the robot to confirm 'stop'. Check the connection. |
Notes for the grown-up
This lesson connects to the Robot or Not ethics rows. Kids are surprisingly good at inventing safety rules once they see three real ones. Keep the wall test slow (speed 3 or 4).
Ages: K-5: the let-go test and the wall test, with a soft box as the wall. 6-12: discuss failure modes (sensor dead, signal lost, software stuck) and which stop catches which.
Materials: Robot with IR receiver and laser, IR remote, a box or soft wall, tape line, app.